Continental drift
Continental drift is the hypothesis, originating in the early 20th century, that Earth's continents move or drift relative to each other over geologic time.[1] The hypothesis of continental drift has since been validated and incorporated into the science of plate tectonics, which studies the movement of the continents as they ride on plates of the Earth's lithosphere.[2]
The speculation that continents might have "drifted" was first put forward by Abraham Ortelius in 1596. A pioneer of the modern view of mobilism was the Austrian geologist Otto Ampferer.[3][4] The concept was independently and more fully developed by Alfred Wegener in his 1915 publication, "The Origin of Continents and Oceans".[5] However, at that time the hypothesis was rejected by many for lack of any motive mechanism. The English geologist Arthur Holmes later proposed mantle convection for that mechanism.
History
Early history
Abraham Ortelius (Ortelius 1596),[6] Theodor Christoph Lilienthal (1756),[7] Alexander von Humboldt (1801 and 1845),[7] Antonio Snider-Pellegrini (Snider-Pellegrini 1858), and others had noted earlier that the shapes of continents on opposite sides of the Atlantic Ocean (most notably, Africa and South America) seem to fit together.[8] W. J. Kious described Ortelius' thoughts in this way:[9]
Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves if someone brings forward a map of the world and considers carefully the coasts of the three [continents]."
In 1889, Alfred Russel Wallace remarked, "It was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had, again and again, changed places with each other."[10] He quotes Charles Lyell as saying, "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages."[11] and claims that the first to throw doubt on this was James Dwight Dana in 1849.
In his Manual of Geology (1863), Dana wrote, "The continents and oceans had their general outline or form defined in earliest time. This has been proved with regard to North America from the position and distribution of the first beds of the
This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger expedition, 1872–1876, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore on what is now known as the continental shelf. This suggested that the oceans were a permanent feature of the Earth's surface, rather than them having "changed places" with the continents.[10]
Eduard Suess had proposed a supercontinent Gondwana in 1885[15] and the Tethys Ocean in 1893,[16] assuming a land-bridge between the present continents submerged in the form of a geosyncline, and John Perry had written an 1895 paper proposing that the Earth's interior was fluid, and disagreeing with Lord Kelvin on the age of the Earth.[17]
Wegener and his predecessors
Apart from the earlier speculations mentioned above, the idea that the American continents had once formed a single landmass with Eurasia and Africa was postulated by several scientists before Alfred Wegener's 1912 paper.[5] Although Wegener's theory was formed independently and was more complete than those of his predecessors, Wegener later credited a number of past authors with similar ideas:[18][19] Franklin Coxworthy (between 1848 and 1890),[20] Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907)[21] and Frank Bursley Taylor (1908).[22]
The similarity of southern continent geological formations had led
Continental drift without expansion was proposed by Frank Bursley Taylor,[26] who suggested in 1908 (published in 1910) that the continents were moved into their present positions by a process of "continental creep",[27][28] later proposing a mechanism of increased tidal forces during the Cretaceous dragging the crust towards the equator. He was the first to realize that one of the effects of continental motion would be the formation of mountains, attributing the formation of the Himalayas to the collision between the Indian subcontinent with Asia.[29] Wegener said that of all those theories, Taylor's had the most similarities to his own. For a time in the mid-20th century, the theory of continental drift was referred to as the "Taylor-Wegener hypothesis".[26][29][30][31]
Alfred Wegener first presented his hypothesis to the German Geological Society on 6 January 1912.[5] His hypothesis was that the continents had once formed a single landmass, called Pangaea, before breaking apart and drifting to their present locations.[32]
Wegener was the first to use the phrase "continental drift" (1912, 1915)
Rejection of Wegener's theory, 1910s–1950s
Although now accepted, the theory of continental drift was rejected for many years, with evidence in its favor considered insufficient. One problem was that a plausible driving force was missing.[1] A second problem was that Wegener's estimate of the speed of continental motion, 250 cm/year, was implausibly high.[34] (The currently accepted rate for the separation of the Americas from Europe and Africa is about 2.5 cm/year).[35] Furthermore, Wegener was treated less seriously because he was not a geologist. Even today, the details of the forces propelling the plates are poorly understood.[1]
The English geologist Arthur Holmes championed the theory of continental drift at a time when it was deeply unfashionable. He proposed in 1931 that the Earth's mantle contained convection cells which dissipated heat produced by radioactive decay and moved the crust at the surface.[36] His Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.[37]
Geological maps of the time showed huge land bridges spanning the Atlantic and Indian oceans to account for the similarities of fauna and flora and the divisions of the Asian continent in the Permian period, but failing to account for glaciation in India, Australia and South Africa.[38]
The fixists
David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could prove there was a force that could move continents, then he might think about it. The idea was moonshine, I was informed."[48]
As late as 1953—just five years before Carey[49] introduced the theory of plate tectonics—the theory of continental drift was rejected by the physicist Scheidegger on the following grounds.[50]
- First, it had been shown that floating masses on a rotating geoid would collect at the equator, and stay there. This would explain one, but only one, mountain building episode between any pair of continents; it failed to account for earlier orogenic episodes.
- Second, masses floating freely in a fluid substratum, like icebergs in the ocean, should be in isostatic equilibrium (in which the forces of gravity and buoyancy are in balance). But gravitational measurements showed that many areas are not in isostatic equilibrium.
- Third, there was the problem of why some parts of the Earth's surface (crust) should have solidified while other parts were still fluid. Various attempts to explain this foundered on other difficulties.
Road to acceptance
From the 1930s to the late 1950s, works by
In 1947, a team of scientists led by
Meanwhile, scientists began recognizing odd magnetic variations across the ocean floor using devices developed during World War II to detect submarines.[57] Over the next decade, it became increasingly clear that the magnetization patterns were not anomalies, as had been originally supposed. In a series of papers in 1959–1963, Heezen, Dietz, Hess, Mason, Vine, Matthews, and Morley collectively realized that the magnetization of the ocean floor formed extensive, zebra-like patterns: one stripe would exhibit normal polarity and the adjoining stripes reversed polarity.[58][59][60] The best explanation was the "conveyor belt" or Vine–Matthews–Morley hypothesis. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. The new crust is magnetized by the Earth's magnetic field, which undergoes occasional reversals. Formation of new crust then displaces the magnetized crust apart, akin to a conveyor belt – hence the name.[61]
Without workable alternatives to explain the stripes, geophysicists were forced to conclude that Holmes had been right: ocean rifts were sites of perpetual orogeny at the boundaries of convection cells.[62][63] By 1967, barely two decades after discovery of the mid-oceanic rifts, and a decade after discovery of the striping, plate tectonics had become axiomatic to modern geophysics.
In addition,
Modern evidence
Geophysicist Jack Oliver is credited with providing seismologic evidence supporting plate tectonics which encompassed and superseded continental drift with the article "Seismology and the New Global Tectonics", published in 1968, using data collected from seismologic stations, including those he set up in the South Pacific.[69][70] The modern theory of plate tectonics, refining Wegener, explains that there are two kinds of crust of different composition: continental crust and oceanic crust, both floating above a much deeper "plastic" mantle. Continental crust is inherently lighter. Oceanic crust is created at spreading centers, and this, along with subduction, drives the system of plates in a chaotic manner, resulting in continuous orogeny and areas of isostatic imbalance.
Evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal fossils are found around the shores of different continents, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus in rocks of the same age at locations in Africa, India, and Antarctica.[71] There is also living evidence, with the same animals being found on two continents. Some earthworm families (such as Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa.
The complementary arrangement of the facing sides of South America and Africa is an obvious and temporary coincidence. In millions of years,
The widespread distribution of
See also
- Geological history of Earth – The sequence of major geological events in Earth's past
- Israel C. White
Citations
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- ^ Oreskes 2002, p. 324.
- ^ Kalliope Verbund: Ampferer, Otto (1875–1947)
- ^ Helmut W. Flügel: Die virtuelle Welt des Otto Ampferer und die Realität seiner Zeit. In: Geo. Alp., Vol. 1, 2004.
- ^ a b c d Wegener, Alfred (6 January 1912), "Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage" (PDF), Petermanns Geographische Mitteilungen, 63: 185–195, 253–256, 305–309, archived from the original (PDF) on 4 October 2011.
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- ^ a b Schmeling, Harro (2004). "Geodynamik" (PDF) (in German). University of Frankfurt.
- ^ Brusatte, Stephen, Continents Adrift and Sea-Floors Spreading: The Revolution of Plate Tectonics (PDF), archived (PDF) from the original on 3 March 2016, retrieved 16 May 2016
- ISBN 978-0-16-048220-5, archivedfrom the original on 8 April 2011, retrieved 29 January 2008
- ^ a b Wallace, Alfred Russel (1889), "12", Darwinism …, Macmillan, p. 341
- ^ Lyell, Charles (1872), Principles of Geology ... (11 ed.), John Murray, p. 258, archived from the original on 6 April 2016, retrieved 16 February 2015
- ^ Antonio Snider-Pellegrini, La Création et ses mystères dévoilés (Creation and its mysteries revealed) (Paris, France: Frank et Dentu, 1858), plates 9 and 10 Archived 5 February 2017 at the Wayback Machine (between pages 314 and 315).
- ^ Dana, James D. (1863), Manual of Geology, Theodore Bliss & Co, Philadelphia, p. 732, archived from the original on 15 May 2015, retrieved 16 February 2015
- ^ Oreskes 2002
- ^ Eduard Suess, Das Antlitz der Erde (The Face of the Earth), vol. 1 (Leipzig, (Germany): G. Freytag, 1885), page 768. From p. 768: "Wir nennen es Gondwána-Land, nach der gemeinsamen alten Gondwána-Flora, ... " (We name it Gondwána-Land, after the common ancient flora of Gondwána ... )
- ^ Edward Suess (March 1893) "Are ocean depths permanent?" Archived 5 February 2017 at the Wayback Machine, Natural Science: A Monthly Review of Scientific Progress (London), 2 : 180- 187. From page 183: "This ocean we designate by the name "Tethys", after the sister and consort of Oceanus. The latest successor of the Tethyan Sea is the present Mediterranean."
- ^ Perry, John (1895) "On the age of the earth", Nature, 51 : 224–227 Archived 17 February 2015 at archive.today, 341–342, 582–585.
- ^ ISBN 978-0-486-61708-4
- ^ Wegener, A. (1929), Die Entstehung der Kontinente und Ozeane (4 ed.), Braunschweig: Friedrich Vieweg & Sohn Akt. Ges.
- ^ Coxworthy, Franklin (1924). Electrical Condition; Or, How and where Our Earth was Created. J.S. Phillips. Retrieved 6 December 2014.
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- ^ Frank Bursley Taylor (3 June 1910) "Bearing of the Tertiary mountain belt on the origin of the earth's plan", Bulletin of the Geological Society of America, 21 : 179–226.
- ^ Mantovani, R. (1889), "Les fractures de l'écorce terrestre et la théorie de Laplace", Bull. Soc. Sc. Et Arts Réunion: 41–53
- ^ Mantovani, R. (1909), "L'Antarctide", Je M'instruis. La Science Pour Tous, 38: 595–597
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- ^ ISBN 978-0-231-53845-9. Archivedfrom the original on 3 June 2016. Retrieved 20 October 2015.
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- ^ R. M. Wood, Coming Apart at the Seams Archived 14 May 2016 at the Wayback Machine, New Scientist, 24 January 1980
- ^ "Wegener and his proofs". Archived from the original on 5 May 2006.
- ^ "Plate Tectonics: The Rocky History of an Idea". Archived from the original on 11 April 2011. Retrieved 23 August 2006.
Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories.
- ^ University of California Museum of Paleontology, Alfred Wegener (1880–1930) Archived 8 December 2017 at the Wayback Machine (accessed 30 April 2015).
- ^ Unavco Plate Motion Calculator Archived 25 April 2015 at the Wayback Machine (accessed 30 April 2015).
- (PDF) from the original on 9 October 2019. Retrieved 15 January 2014.
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- ^ See map based on the work of the American paleontologist Charles Schuchert in Wells, H. G.; Huxley, Julian; Wells, G. P. (1931), The Science of life, p. 445
- ^ Şengör (1982), p. 30
- ^ Şengör (1982), p. 28
- ^ Şengör (1982), p. 29
- ^ Şengör (1982), p. 31
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- ^ a b c d e Frankel (2012), p. 403
- ^ a b Frankel (2012), p. 405
- ^ Frankel (2012), p. 407
- ^ Frankel (2012), p. 409
- ^ McKie, Robin (28 October 2012). "David Attenborough: force of nature". The Observer. London. Archived from the original on 31 October 2013. Retrieved 29 October 2012.
- ^ Carey, S. W. (1958). Carey, S. W. (ed.). "Continental Drift—A symposium". Hobart: Univ. of Tasmania. pp. 177–363.
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- ^ Wessel, P.; Müller, R. D. (2007), "Plate Tectonics", Treatise on Geophysics, vol. 6, Elsevier, pp. 49–98
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- ^ Lippsett, Laurence (2001). "Maurice Ewing and the Lamont–Doherty Earth Observatory". Living Legacies. Archived from the original on 12 January 2018. Retrieved 4 March 2008.
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- ^ "Victor Vacquier Sr., 1907–2009: Geophysicist was a master of magnetics", Los Angeles Times: B24, 24 January 2009, archived from the original on 8 January 2014, retrieved 20 May 2018.
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- ^ Blakemore, Erin (30 August 2016). "Seeing Is Believing: How Marie Tharp Changed Geology Forever". Smithsonian.
- ^ Evans, R. (November 2002). "Plumbing Depths to Reach New Heights". Retrieved 2 June 2008.
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- ^ Wills, Matthew (8 October 2016). "The Mother of Ocean Floor Cartography". JSTOR. Retrieved 14 October 2016. While working with the North Atlantic data, she noted what must have been a rift between high undersea mountains. This suggested earthquake activity, which then [was] only associated with [the] fringe theory of continental drift. Heezen infamously dismissed his assistant's idea as "girl talk." But she was right, and her thinking helped to vindicate Alfred Wegener's 1912 theory of moving continents. Yet Tharp's name isn't on any of the key papers that Heezen and others published about plate tectonics between 1959–1963, which brought this once-controversial idea to the mainstream of earth sciences.
- ^ "Jack Oliver, Who Proved Continental Drift, Dies at 87". The New York Times. 12 January 2011. p. A16. Archived from the original on 26 May 2013. Retrieved 6 June 2013.
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- ^ "Rejoined continents [This Dynamic Earth, USGS]". USGS. Archived from the original on 25 August 2010. Retrieved 22 July 2010.
General and cited sources
- Frankel, Henry R. (2012). The Continental Drift Controversy. Vol. I: Wegener and the Early Debate. Cambridge.
- Le Grand, Homer Eugene (1988). Drifting Continents and Shifting Theories. Cambridge University. ISBN 978-0-521-31105-2.
- ISBN 978-0-19-511732-5. (pb: 0-19-511733-6)
- Oreskes, Naomi (2002). "Continental Drift" (PDF). In Munn, Ted; MacCracken, Michael C.; Perry, John S. (eds.). Encyclopedia of Global Environmental Change. Vol. 1. Chichester, West Sussex: John Wiley & Sons. pp. 321–325. OCLC 633880622. Archived from the original(PDF) on 4 February 2012.
- Ortelius, Abraham (1596) [1570]. Thesaurus Geographicus (in Latin) (3 ed.). Antwerp: Plantin. OCLC 214324616. (First edition published 1570, 1587 edition online)
- ISBN 978-0-471-103769.
- Snider-Pellegrini, Antonio (1858). La Création et ses mystères dévoilés. Paris: Frank and Dentu..
External links
- Benjamin Franklin (1782) and Ralph Waldo Emerson (1834) noted Continental Drift
- A brief introduction to Plate Tectonics, based on the work of Alfred Wegener
- Animation of continental drift for last 1 billion years
- Maps of continental drift, from the Precambrian to the future
- 3D visualization of what did Earth look like from 750 million years ago to present (at present location of your choice)